<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>09 模态窗</title>
  <script>
    window.onload = function () {
      var login = document.getElementById("login");
      var mask = document.getElementById("mask");
      var box = document.getElementById("box");
      login.onclick = function () {

      };
      getElementsByClassName()
    }
  </script>
  <style>
    .mask {
      width: 100%;
      height: 100%;

    }
  </style>
</head>
<body>
<a href="#" id="login">login</a>
<div class="mask">
  <div class="box">

  </div>
</div>
</body>
</html>